Just a couple other points. In a "Non conflict" situation a "block" is merely a "contest" if it's set up by active opposition or a "simple action to overcome" if it is already there in the scene.
In a "Extended Contest" a "block" is just part of the opposed rolls, with flavor being to impede you instead of to advance the cause of the NPC. It just adds shifts to the enemy overall total for success. This can be a consequential conflict, which raises the stakes a bit (eg, bypassing the crowd contest difficulty set by the opposition with intimidation, athletics or might could cause a minor social consequence, where dealing with it via a "softer" social skill might not)
Blocks as actual blocks defined are only in "Conflicts" (YS 199, bolding is mine)
Block (YS199): Roll to set up a preemptive defense
against a specified future action; anyone
committing that named action will have to roll
against the block to succeed (page 210).
YS210
To perform a block, declare what specific type
of action the block is intended to prevent and
roll an appropriate skill. The total of that roll is
called the block strength.
Action: Shapeshift out of human form
Appropriate Skill: Presence (keep somebody important focused on interacting with the shapeshifter), Rapport (make the person prefer human form because an attractive person is making you feel good about your appearance), Intimidate (threaten to hurt them or somebody they care about if they change), Discipline+evocation (use mind magic to lock down their ability to shapeshift), lore(complexity)+thaumaturgy (skill-like ability to block shapeshift, requires connection to the target + ritual)
During the exchange,
any time a character wants to perform the action
that’s covered by the block, he must roll against
the block and meet or exceed the block strength
to be able to perform that action.
Examples of what to roll against the block in above examples: Deceit (distract the individual watching you), Discipline (ignore your hormones and shift), various social defenses (discipline to work through the fear, or in case of threat to others, you might counter-intimidate or invoke contacts or something to defuse the threat), Discipline (mental defense against evocation), Discipline (mental defense) or maybe Lore (to break the connection or find a loophole in the thaumaturgy)
When you create
a block, the block has to be specific and clear in
two ways: who it’s intended to affect, and what
types of action (attack, block, maneuver, move)
it’s trying to prevent. Generally speaking, if the
block can affect more than one person, it can
only prevent one type of action. If the block only
affects one person, it can prevent several types of
action—up to all of them—as context permits.
You can’t use a block to prevent someone from
making a defense roll.
Given that you can clearly block other supplemental or free actions (you can prevent people from speaking, drawing a weapon, etc), shapeshift shouldn't be any different. It certainly is specific enough.
An area block against shapeshift is unlikely to work, but isn't impossible (indeed Dresden did exactly that when he dispelled the magic of the lycanthropes when they were trying to gather it in one scene of the 2nd book). Certainly intimidate might get it done.
Keep in mind that there are some blocks that
just won’t work in some situations. (Trying the
“keep them pinned down with gunfire” trick on
a loup-garou isn’t going to really help you much,
given that they’re immune to bullets.)
This covers the "fly over walls, shapeshift into mist, Molly leaves an image while sneaking off, you aren't actually a vampire so brandishing a cross doesn't matter" situations. No rolling, the block just won't work against the technique used to ignore it.
You'll note there is nothing that says anything about a block against a "roll". It is against an action. Likewise it says nothing about what skill is used to defeat a block. It depends on the nature of the block and yes, a block against shapeshift (or anything else) the player trying to overcome the block gets total freedom on how to get out of it...but his solution must make sense to the table, preferably improving the story.
Just think about how many skills and powers can be used to bypass a wall, and how different a set of skills and powers might bypass a crowd on a dance floor. The consequences of some approaches might be permanent (destroying the wall or using an explosion to clear the dance floor) and might cause aspect-compels or long term consequences.